1. Field of the Invention
The present invention relates to a curve length measuring apparatus and particularly to a pen-holder type curve length measuring apparatus having a castor mechanism so as to be able to trace an object curved line correctly and easily.
2. Prior Art
There are curve length measuring apparatus used for measuring the length of curves on maps, drawings and the like. Generally, these apparatuses are characterized in being used readily like drawing with a pencil. Therefore, most of the apparatuses have a simple construction typically comprising a pen-holder like body, a rolling disk provided at the tip of the body and an indicator mounted on the body. The rolling disk is for detecting the length of a curved or straight line by rolling the disk along the curve or the line. Since the axis of the rolling disk is rigidly connected with the body, while an operator traces a curve with the rolling disk, the operator has to continue to rotate the body in the tangential direction of the curve, so it is difficult to trace curves correctly whose direction changes in a complicated manner. Further, it is hard to read the indicator due to the ever rotating body. These difficulties lead to incorrect or erroneous measurement of curve length.
Moreover the cost of such apparatus could be reduced by applying convenient devices.
A caster type apparatus, having an electro-optical component, under utilized, because of short battery life, less shock stability and high cost, as caused by complicated and delicate optical structure of many optical devices.
Accordingly, it is an aim of the present invention to obviate the disadvantages of the prior art and it is an object of the present invention to provide a handy type curved length measuring apparatus capable of tracing curves correctly without rotating the body, i.e., with the body directed towards the operator so as to read the indicator easily, resulting in marketable apparatus.
In order to achieve the object, a curve length measuring apparatus of the present invention comprises: a castor rotatably connected with a body so as to rotate around a first axis of the castor, a rolling disk rotatably connected with castor so as to rotate around a second axis of the rolling disc; a marketable device so called photo interrupter assembled in the castor so as to generate electronic pulses; and an electronic coupling, e.g. a slip ring, forming part of the castor.
In one embodiment, the apparatus further comprises a plurality of slits provided on the rolling disk along the outer periphery thereof; light emitting means for emitting light and light detecting means for light detection installed in the photo interrupter, the light emitted from one side in the air gap of the photo interrupter to the other side where emitted light is directly converted to electricity; the photo interrupter generating electric pulses as the rolling disk rotates; and electric coupling comprising a bush and slip-ring conducting electric current from the body to the rotatable castor; and pulse counting means for indicating a curve length based on the number of pulses.